This invention relates to methods of making aluminum nitride in fine particulate form. In an important specific sense, the invention is directed to methods of making aluminum nitride of very high purity and submicron particle size.
One illustrative field of use for aluminum nitride made by the present methods is in the production of aluminum nitride substrates for chips for electronic applications, e.g. thyristors, diodes, very large scale integration (VLSI) chips, etc., employed in equipment such as computers. At present, such chips predominantly utilize alumina substrates, but the low thermal conductivity of alumina gives rise to problems of overheating in chips that have a high packing density of active devices or otherwise tend to generate relatively large amounts of heat in operation. Beryllia (BeO), proposed as an alternative chip substrate material, has a thermal conductivity about 7 to 8 times that of alumina, but presents other problems owing to its adverse thermal expansion properties, poor adhesion to silicon, toxicity, high cost and low availability.
Aluminum nitride possesses properties potentially favorable for chip substrate use, having a high electrical resistivity, a thermal conductivity as much as 7 to 10 times that of alumina, and a low thermal expansion coefficient. Very advantageously, up to a temperature of about 200.degree. C. its thermal expansion coefficient is in close agreement with that of silicon. Currently commercially available aluminum nitride, however, is too low in purity for use in chip substrates; one difficulty with low-purity material arises from the fact that the thermal conductivity of aluminum nitride is directly related to material purity. Also, aluminum nitride bodies suitable for substrate use are made by sintering very fine particulate aluminum nitride, and known procedures do not enable convenient and economical attainment of material of adequate purity and suitably fine particle size for this purpose.
By way of illustration, a conventional process for aluminum nitride manufacture is the carbothermic reduction of alumina followed by nitridation. The product is aluminum nitride of approximately 98.5-99.0% purity, the remaining 1-1.5% being unreacted alumina and carbon. This and other known processes require high purity starting materials, and their products must be crushed and ground in order to achieve aluminum nitride of sufficiently fine particle size for sintering to form chip substrates. The necessity of crushing and grinding adds to processing costs, and results in contamination of the ground particulate nitride with foreign matter such as grinding media.
It is reported that submicron high purity AlN has been made in a plasma reactor by direct reaction of Al vapor and nitrogen gas at a temperature of about 7000.degree. K. However, the process is very expensive, has a low efficiency, is associated with technical problems and has never been commercialized.
A similar product is also reported as having been made by direct reduction of AlCl.sub.3 with gaseous ammonia at a temperature within the range 500.degree.-1200.degree. C. The reaction is endothermic and therefore energy-expensive. It is difficult to obtain AlCl.sub.3 of the requisite high purity, and that used in the reaction is totally consumed and not available for recycling. The process is uneconomical and, as far as the present applicants are aware, has never been used commercially.
U.S. Pat. Nos. 3,477,812 and 3,607,014 describe a procedure for producing aluminum nitride single crystals by contacting a gaseous monovalent aluminum halide such as aluminum monochloride (i.e., AlCl) with a gaseous nitriding agent such as N.sub.2 while providing an inert surface on which the crystals can form. In an example given in these patents, the AlCl is formed by reaction of Al.sub.4 C.sub.3, CaCl.sub.2 and C and is reacted with N.sub.2 at a temperature of about 1700.degree. C. to produce AlCl.sub.3 gas and AlN crystals; the crystals are said to range generally in diameter from about 1 to about 15 microns and generally to have a length-to-diameter ratio of about 500 to about 2500. These relatively large crystals would not be sinterable to produce chip substrates.